MEMS force sensors are used to detect small input force that ranges from micro-Newtons to milli-Newtons. Two main technologies are used to detect mechanical force in such scales: piezoresistive and capacitive force sensors. Piezoresistive force sensors offer several advantages such as simple design of the sensor and simple readout circuit. However, they are sensitive to the temperature changes. An example of such sensors is presented in the U.S. Provisional Patent Application No. 2014/0007705 A1, which is used to detect touch force of the user finger on display screens. Capacitive force sensors provide accurate measurements at micro-Newton ranges and they are not sensitive to temperature changes, but when force range increases the sensitivity of such sensors is compromised.
The present invention of MEMS piston-tube capacitive force sensor is able to overcome the limitations of conventional MEMS capacitive force sensors. The sensor is able to detect small forces in the range of few micro-Newtons as well as large input forces in the range of 10's of milli-Newtons. This is due to the high sensitivity of the piston-tube electrode configuration that enables the detection of small displacements of the rotor of the sensor in response to the input force, and to the large stroke that the rotor can travel, and to the high stiffness of the flexure mechanism of the rotor.
MEMS piston-tube capacitive sensor force can also withstand over limit input force, which makes such sensors useful for user input devices such touch screen or hand held devices that require an input in the form of touch force applied by the finger of a user or by a stylus.